1. Field of the Invention
The present invention relates to an improved ladder hinge and locking mechanism, and more particularly to a ladder hinge that has a plurality of locking positions which enable displacement of attached ladder stringers from a closed, storage position, through a step-ladder configuration, to an extended, straight ladder position.
2. Prior Art
In an earlier patent of the same inventor, U.S. Pat. No. 4,407,045, an extensive discussion of the prior art for multi-positioned ladder hinge locking mechanisms was discussed. The present invention constitutes an improvement over the structure which was disclosed in this earlier patent (referred to hereafter as the '045 patent). This patent discloses a rotating and locking mechanism for joining two hinge members rotationally about a common axis. The respective hinge members are mounted on a hub with one of the hinge members being fixed to rotate with the hub, while the remaining member is free to rotate independently of the hub structure. The hub houses a shaft which is oriented along the rotational axis for the hinge and which is co-axial with the longitudinal axis of the hub. A disk is attached at one end of the shaft and a handle with locking pins is positioned at the remaining shaft end. A plurality of locking apertures are diametrically positioned for selective alignment with the locking pins, depending upon the particular position desired for the ladder stringers.
The core opening or aperture for the hub is constructed with two differing inner diameters. The larger inner diameter of the hub is located on the distal side from the handle and locking pins. The lesser diameter is more proximate to the handle and establishes a blocking shoulder to restrain axial movement of the disk as the handle is extended away from the hinge members. The blocking shoulder prevents the locking pins from being fully removed from the outer hinge member, while enabling the inner hinge member to rotate to a new adjustment position. The patent also describes a detent arrangement which enables the handle to lock to a temporary extended position during rotation of the hinge to a new adjustment. The handle and locking pins are biased to an inserted, locking position by a spring which is enclosed within the hub core and between the disk and an opposing wall through which the shaft extends to the handle.
The detent mechanism is required to hold the handle in an extended position because of the awkward coordination of movements associated with moving the ladder configuration while pulling the handle and locking pins free from the rotating hinge members. For example, it is apparent to those skilled in the art that without the detent arrangement which holds the handle in its extended configuration, a person manipulating the ladder must both hold laterally on the handle located at the top of the ladder, while at the same time pushing apart on the legs or stringers which are attached to the hinge. A concurrent opposing force arising during this push-pull sequence creates substantial difficulty for the user in making regular adjustments to ladder positions. Furthermore, the user has limited leverage with respect to the ladder because of the required pulling action in order to release the locking pins from the rotatable hinge. This reduced leverage associated with the pulling movement further adds to the inconvenience of operating the adjustable locking mechanism for the '045 patent. Furthermore, the structural requirement that the biasing spring be placed within the hub severely limits the size of spring which can be applied to the adjustable locking hinge.
In addition to the safety and convenience considerations, it should be noted that the hub structure requires several tooling operations to develop the differing diameters making up the hub core or aperture. In addition, the retaining wall opposite the disk further adds to the increased cost of manufacture and expense of the hub and related structure. Because of the significantly lower cost for ladders which do not use a locking hinge similar to the referenced prior art devices, each aspect of increased cost increases consumer reluctance to purchase an adjustable, locking ladder, despite the many advantages which such a device provides.